The present invention relates to a method for judging a monitoring condition for diagnosis of an engine and more specifically relates to a method for judging whether or not an engine is in a steady state operation during which the monitoring for diagnosis is performed.
Recently, with an increase of vehicles utilizing computer-based powertrain control systems, more and more vehicles are equipped with a self-diagnostic or an on-board type diagnostic system capable of monitoring malfunctions of devices or systems, such as a catalytic converter, oxygen sensors, an ignition system, miscellaneous actuators and other sensing equipments installed on vehicles.
In monitoring those devices or systems, the most important thing is to determine the engine operating condition in which the monitoring is properly performed and the period during which the data are collected into the diagnostic system.
For example, Japanese Patent Application Laid-Open No. 1987-172221 discloses a diagnostic technology in which the diagnosis is executed by simply comparing the output level with the threshold level. However, this diagnostic technology often tends to provide erroneous judgements when the engine is in a transient condition in which the engine operational condition changes momentarily. That is to say, in order to obtain reliable results constantly, it is necessary to monitor the object device under a stable condition as far as possible excepting in some peculiar cases.
The stable condition described here is defined as a steady state operation of an engine in which an engine rotational speed is constant and at the same time among the engine parameters of an intake air amount, an intake manifold pressure, a fuel injection amount, and a throttle opening angle, at least one engine parameter is also constant.
Further, especially when it is needed to detect a malfunction derived from the deterioration of the device itself, a certain period of monitoring is absolutely necessary.
The prior method of judging whether an engine parameter like an engine speed is constant or not when judging a steady state operation has to check whether or not the engine parameter exists between an upper limit and a lower limit, and at the same time whether the variation of the engine parameter is within a predetermined band width.
In general, the range of the steady state operation becomes more accurate and therefore the result of diagnosis becomes more reliable, if the above upper and lower limits and the predetermined band width are established in a smaller size.
However, if the above limits or the band width are too small, it happens that the diagnosis would not start due to no chance of the steady state judgement, or the diagnosis would discontinue halfway due to a very short monitoring period.
Further, there is limitation in the accuracy of a sensor or the processing capability of the CPU when those limits or the band width are attempted to be made small.